1. Field of the Invention
The present invention relates to an imaging device configured to form an image on a solid-state image pickup device, in particular to miniaturization of such an imaging device which includes an on-axis lens element and a radially-retractable lens element, wherein the on-axis lens element remains on an imaging optical axis (photographing optical axis) and the radially-retractable lens element is movable between an on-axis position in which the radially-retractable lens element is positioned on the imaging optical axis and an off-axis position in which the radially-retractable lens element is retracted away from the imaging optical axis.
2. Description of the Related Art
Miniaturization of cameras has been in increasing demand. Above all, further miniaturization of retractable photographing lenses, specifically the length thereof in a non-operating state, has been in strong demand. To meet such demands, the inventors of the present invention have proposed a retractable photographing lens disclosed in U.S. Pat. No. 6,978,089 (U.S. patent application Ser. No. 10/368,342) in which an optical element (displaceable element) of an imaging optical system (photographing optical system) is retracted to a radially-retracted position deviating from the imaging optical axis of the imaging optical system (i.e., displaced from the imaging optical axis of the imaging optical system), and at the same time, the optical element (together with other optical elements of the imaging optical system) is retracted toward a picture plane when the photographing lens is fully retracted. More specifically, the imaging optical system includes an on-axis lens element and a radially-retractable (displaceable) lens element, wherein the on-axis lens element remains on the imaging optical axis and the radially-retractable lens element is movable between an on-axis position in which the radially-retractable lens element is positioned on the imaging optical axis and an off-axis position in which the radially-retractable lens element is radially retracted away from the imaging optical axis. The radially-retractable lens element is positioned on the imaging optical axis when the photographing lens is in use (in a ready-to-photograph state), and is radially retracted away from the imaging optical axis when the photographing lens is retracted.
An image pickup device of this type of photographing lens is usually a solid-state image sensor, and the imaging surface (light-sensitive surface) thereof is rectangular in plan view that includes a short-side direction and a long-side direction. On the other hand, each of the on-axis lens element and the retractable lens element is conventionally circular in plan view, and the radially-retractable lens element is supported by the free end of a swingable member pivoted about a pivot shaft. Conventionally, the off-axis position of the radially-retractable lens element is set on a straight line which passes through the imaging optical axis of the on-axis lens element and extends in the short-side direction of the solid-state image sensor (i.e., extending parallel to the short sides of the solid-state image sensor), while the pivot shaft of the swingable member is positioned between the on-axis position and the off-axis position of the radially-retractable lens element on one of the opposite sides (areas) of the on-axis lens element in the long-side direction of the solid-state image sensor (i.e., the pivot shaft is positioned along the short side of the solid-state image sensor). However, in this conventional structure, the structure around the radially-retractable lens element has not been sufficiently miniaturized and the diameter of this structure has not been sufficiently reduced either, and accordingly, there is room for further miniaturization of the structure around the radially-retractable lens element and a further reduction in diameter of the same structure.